Alternative NVM technologies require new test approaches, part 2

Editorís note: this is part two of an ongoing series on testing memory.

In Part 1 of this article, I outlined the growing concern among manufacturers of consumer products that incorporate memory devices that floating-gate flash memory would one day soon no longer be able to satisfy their requirements and the search for alternative NVM technologies. I discussed one alternative to flash memory, phase-change memory (PCM), and explored emerging device characterization approaches. Part 2 addresses the testing challenges associated with another emerging NVM technology, ferroelectric memory (FRAM).

Ferroelectric MemoryFRAM relies on charge storage in a capacitor but uses a ferroelectric layer instead of the dielectric layer of a typical capacitor. The memory mechanism for FRAM is based on polarization shift in ferroelectric materials. Ferroelectric materials have strong non-linear dependency between the applied electrical field (E) and polarization (P). When the electric field reaches a critical level, the ions inside the crystalline structure move from one stable location to another. This shift is accompanied by a shift of the ferroelectric domain walls. Electrically, it is represented by the hysteresis chart (see figure 9), showing the dependency between electrical field and polarization. The switch between one state and another is characterized by the area of hysteresis, which represents the amount of charge moved during re-polarization.

Click image to enlarge.

Figure 9: Hysteresis curve generated and measured by the Model 4225-PMU with Model 4225-RPMs shows the variation in the polarity charge as the voltage across the material varies. Ec is the coercive field and Pr is the remnant polarization, which are key parameters for FRAM performance. Proper test parameters on a good, non-leaky device should show a complete loop, with the beginning and end at 0 V.

The challenge of characterizing the ferroelectric capacitor is that the fundamental behavior is switching of the polarization state of the ferroelectric material, which requires measuring the polarization charge on the capacitor as it changes. Typically, a load capacitor, pulse generator, and oscilloscope are used, usually in a Sawyer-Tower circuit. In this approach, measuring the transient voltage using an oscilloscope or digitizer across the load capacitor is a proxy for the charge flowing into the ferroelectric material. This method has several drawbacks, however.

The load capacitance must be relatively large compared to the ferroelectric capacitance to ensure that the voltage drop across the load capacitor is not significant; otherwise, various undesirable assumptions or approximations have to be applied to obtain the voltage across the ferroelectric element. This large load capacitance means that the sense voltage is fairly small.

This small voltage is difficult to measure accurately with an oscilloscope or digitizer. A better solution is to use an instrument that can measure current and voltage directly and simultaneously so that the total charge can be accurately determined without the need for a load capacitor. The charge is calculated from the high-speed current measurements, which are sampled consistently over time.